Italian patent 1 207 630 discloses equipment for teaches the assembly of a filter bag coated with a film of heat-sealable thermoplastic material and having a label attached by a thread that is anchored at one end of the bag between two heat-sealed edges, then passed around and secured externally to the bag by a number of spot seals.
The manufacture of this type of bag dictates the use, at least to an extent, of paper or other material such as can be heat-sealed. Unfortunately there are certain considerations precluding the use of such materials, connected with the risk that substances released from the heat-sealed thermoplastic can migrate into the infusion following interaction with a liquid at high temperature.
A further drawback of this solution is discernible in the higher cost of heat-sealable paper as compared to normal filter paper.
U.S. Pat. No. 2,307,998 discloses a solution, belonging to the prior art, wherein the thread, the bag, and the finger tab label are interconnected by knots.
The formation of knots as taught by that patent does not favor automation of the manufacturing process to a speed that would be suitable for the industrial scale production of filter bags containing an infusionable substance; indeed as regards this aspect of the process, the industrial solution most widely adopted remains that of securing the thread with metal staples.
There is also a drawback with the solution of stapling, however, in that it introduces a foreign element into the infusion of which the presence is considered inappropriate for obvious reasons of hygiene. Moreover, the presence of a metal staple is undesirable by reason of the possible change in organoleptic properties of the infusion attributable to the metal.
Italian patent application n.sup.o B094A 000319, filed by I.M.A. INDUSTRIA MACCHINE AUTOMATICHES S.p.A., discloses a method of attaching a label to a filter bag by means of a special knot such as will allow the manufacture of bags, with respective finger tab labels, on industrial scale, and making no use of metal staples.